Structural Distinctions Between Biogenic and Geological Aragonite
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Structural Distinctions Between Biogenic and Geological Aragonite Boaz Pokroy, John P. Quintana1 and Emil Zolotoyabko Department of Materials Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel. 1 DND-CAT Research Center, Northwestern University, APS/ANL Sector 5, Building 432A, 9700 South Cass Avenue, Argonne, IL 60439-4857, U.S.A.
ABSTRACT Energy-variable synchrotron x-ray diffraction was applied to probe local structure in different kinds of aragonitic seashells with spatial resolution. All investigated specimens revealed lattice distortions of biogenic aragonite with respect to the geological mineral. The obtained data are analyzed in terms of residual strains induced by intercalating organic macromolecules.
INTRODUCTION In the last few decades, the biomineralization process has been intensively studied in order to clarify the interrelation between the structure of biogenic crystals and their functioning [1,2]. In particular, great attention has been given to mollusk seashells because of their outstanding mechanical characteristics, which are achieved within hierarchically ordered and lightweight structures [3-6]. In fact, the seashells are composite materials comprising ceramic matrix and a 0.1 - 5 wt% of organic phase, which is mainly located within the inter-crystalline boundaries. Besides that, some organic macromolecules (intra-crystalline) are presumably intercalated within the crystalline lattice [7,8]. These “buried molecules” are a source of local deformation fields which can strongly affect the mechanical properties of biogenic crystals. Very recently we performed high-resolution synchrotron x-ray diffraction measurements with powdered samples of Acanthocardia tuberculata bivalvia seashells [9], which revealed highly anisotropic lattice distortions in biogenic aragonite as compared with the same mineral of geological origin. These distortions were attributed to the presence of organic molecules within the CaCO3 matrix. In order to obtain more complete information on this phenomenon we performed synchrotron x-ray diffraction measurements with the un-powdered seashell pieces. The structural parameters obtained in the nacre and cross-lamellar layers in two different seashells are summarized below.
EXPERIMENTAL DETAILS In this study we used energy-variable x-ray diffraction [10, 11] on the 5BMD beamline of the Advanced Photon Source (APS) at Argonne National Laboratory in order to measure lattice distortions in different layers of seashells with depth resolution. The accurate tuning of the x-ray energy results in the controlled changes in the x-ray
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penetration depth, Λ, which in the Bragg scattering (reflecting) geometry, is defined by the linear absorption coefficient, µ, and the Bragg angle, ΘB, (both being energy dependent): sin Θ B λ (1) = 2µ 4µd By increasing the x-ray energy, the diffraction signal is obtained from progressively deeper layers, which provides information on depth-dependent structural parameters. In the current study, we used x-ray energies
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